Cathode ray display tube devices are, of course, well known for the production of images. In certain specialist applications, it is desirable that the light from such a cathode ray tube display device should be collimated, that is to say it should be concentrated along a principal axis of the display device with little or no light from the C.R.T. being lost to the sides, at angles to said axis. One way of achieving this effect is to form the front of the cathode ray tube, i.e. the portion providing the screen, as a so-called fibre optic plate in place of the more conventional substantially homogeneous plate of thick glass. Such a fibre optic plate usually comprises a light-transmitting planar plate which behaves as if it comprised a large number of optical fibres, (for example glass fibres), arranged as a "bundle" in which the fibres run generally parallel with each other and generally perpendicular to the major faces of the plate, each end of each fibre terminating in a respective face of the plate with the interstices between the fibres being filled with an opaque medium. Thus, the individual optical fibres form "light pipes" oriented for the conduction of light from one face of the plate to the other generally at right angles to the plane of the plate. Such a fibre optic plate may be formed by forming initially a cylindrical rod of transparent glass surrounded circumferentially by a layer of opaque, black glass fused to the transparent glass, the glass being at a temperature at which it is plastic and drawable, "pulling" the composite rod of glass whilst it is still in this plastic state to increase its length and reduce its diameter, dividing the pulled rod into a plurality of lengths which are placed together in a bundle and drawn again so that the individual lengths fuse together affording an integral body of glass comprising several transparent approximately cylindrical regions extending through an opaque black matrix, dividing the last-noted body again into several lengths which are again placed together and pulled once again, the resultant being cut into lengths, bundled together and pulled again and so on, the number of transparent cylindrical regions being multiplied at each stage and the diameters of said regions being reduced at each stage until the required pitch and diameter of such transparent cylindrical regions are reached. The average diameter of each transparent region or "fibre" at this stage may be around 5.mu. (5.times.10.sup.-6 metre) with concomitant spacing between "fibres". The resulting unitary "log" of fused glass is then allowed to cool at an appropriate rate before being cut into individual slices perpendicularly to the direction in which said transparent regions or "fibres" extend, each such slice forming a respective fibre optic plate. The opposite major faces of each such plate are then polished to optical standard. To incorporate such a fibre optic plate as the front wall of a cathode ray tube, the edges of such a ground and polished slice are cut to the appropriate size and shape to fit a pre-formed shell affording the rear part of the cathode ray tube and the fibre optic plate fused in place or cemented in place using an appropriate resin, the rear face of the fibre optic plate being coated with a phosphor layer before or after such fusing or cementing of the fibre optic plate to the remainder of the tube and with remaining manufacturing stages including evacuation and sealing of the tube, proceeding in the conventional fashion.
It would also be possible to produce a "fibre optic plate" within the meaning of the term as used herein by forming a "log" or bundle of optical fibres of relatively great length, the fibre bundle being impregnated with a selected opaque, preferably black, cement and the bundle being compressed transversely to the longitudinal direction of the bundle prior to curing of the cement to ensure that the individual optical fibres are packed closely together. Fibre optic face plates may then be formed by sawing thin slices from the "log" thus formed and polishing the opposite faces of each plate to optical quality. Conceivably, a similar product could be made using, for example, transparent plastics fibres cemented together by opaque resin.
The collimating effect of such a fibre optic plate is illustrated schematically in FIG. 2 which shows to an enlarged scale and in section, at 20, such a fibre optic plate, the phosphor layer being indicated at 22. Light from the phosphor layer 22, caused by electrons striking the latter, enters the rear face of each optical fibre at various angles, with light rays which enter parallel with the axis of an optical "fibre" 24, (i.e. a transparent region) and thus perpendicular to the major faces of the fibre optic plate, passing straight through the plate, whilst light rays entering more than a very slight angle pass through the sides of the "fibres" and are absorbed by the opaque glass (indicated at 28 in FIG. 2). Accordingly, because of selective transmission of light rays which are directed approximately parallel with the longitudinal axis of the fibres, the light emerging from each fibre 24 is confined to a relatively narrow solid angle centred on the fibre axis, thereby affording an approximation to the desired collimation of the light from the C.R.T. screen. A disadvantage presented by such an arrangement, however, particularly when the C.R.T. image is subjected to magnification, is that the opaque glass between adjoining "fibres" is visible when the screen is viewed normal to the plane of the fibre optic plate, as illustrated diagrammatically in FIG. 3, presenting a so-called "chicken-wire" effect.